The long-range objectives of this research are to provide information concerning the metabolism of the branched chain amino acids and to shed some light on the enzymatic defects in the genetic diseases of man affecting metabolism of these amino acids. We have obtained a preparation of branched chain keto acid dehydrogenase which is approxiamtely 90 percent pure. The enzyme is a multienzyme complex with a molecular weight of approximately 3 million daltons. The complex is composed of at least 3 subunits. One subunit is a decarboxylase with a molecular weight of 35,000. Another subunit which has been identified is lipoamide dehydrogenase with a molecular weight of 53,000. A third subunit with a molecular weight of 49,000, is probably the transacylase. The enzyme is stimulated by L-valine and we have studied the kinetics of the enzyme's activity in the presence and absence of valine. It appears that valine acts as an allosteric effector of the enzyme causing an increased affinity of the enzyme for its keto acid substrates. We have also isolated a number of mutants of P. putida with defects in keto acid dehydrogenase. The object of these was to determine if there is a single lipoamide dehydrogenase for pyruvate, 2-ketoglutarate and branched chain keto acid dehydrogenases. We have isolated a mutant lacking lipoamide dehydrogenase, but it appears to be a polar mutation which affects that entire 2-ketoglutarate dehydrogenase genome. This mutant is being further investigated for its enzymatic activities.